The invention relates generally to nondestructive methods for measuring service-related degradation in protective coatings such as those used to protect gas turbine blades from high temperature oxidation and corrosion. More particularly, the invention is a method and system that uses nonlinear harmonic detection methods to sense degradation-related changes in the magnetic permeability of the high-temperature protective coatings applied to the surface of a workpiece. The invention uses a time-varying magnetic field at a fundamental frequency to detect changes in the magnetic properties of the coatings. The odd-numbered harmonic frequencies are detected and their amplitudes are related to the magnetic permeability of the coating under test to determine coating degradation. By using different fundamental frequencies, it is possible to profile coating degradation with depth and minimize coating thickness effects.
Metallic coatings are commonly used on metal components to protect the surfaces of the components from high-temperature oxidation and corrosion. A common use is on combustion turbines to protect the surfaces of components such as blades. One such class of metallic coatings used for protection typically has the composition MCrAlY, where M may represent either cobalt (Co), nickel (Ni), or a combination of both, Cr represents chromium, Al represents aluminum and Y represents yttrium. Detection of degradation or failure of the metallic coatings is important to prevent damage to the underlying components. During service, the aluminum (AL) in the coating diffuses inward into the base material of the component and outward to form a protective aluminum oxide layer which forms on the outside surface of the coating. Eventually, the aluminum in the coating becomes depleted and can no longer support the aluminum oxide layer. This results in coating failure and lack of protection for the underlying component, such as the blade base metal of combustion turbine components. Coatings can be stripped and replaced, provided this is done before the coating fails. If the protective coating has degraded to the point where it is no longer functional, damage to the underlying component can occur primarily due to high temperature oxidation or corrosion of the material. If the degradation of the coating is not detected prior to coating failure, it may be necessary to replace the entire component.
It has been observed that in some classes of coating systems, degradation causes the coating to change from an initial nonferromagnetic condition to a ferromagnetic condition. By measuring the magnetic properties of the coating, it is possible to nondestructively determine the condition of the coating. Techniques using eddy currents and permeability probes have been applied to this problem. The eddy current measurement technique uses a time-varying magnetic field to induce eddy currents into the component. The eddy current sensing device always generates a reading even if the material is in a nonferromagnetic state (and therefore the coating is not degraded) because the component material is electrically conductive and the eddy current method responds to conductivity as well as magnetic condition. Also, if the thickness of the coating changes, the eddy current measurement will change even if the coating is not degraded. As the coating degrades, the eddy current reading changes only slightly and it is difficult to determine that this small change is due to degradation and not to other factors. The permeability probe uses a permanent magnet which supplies a non-time-varying magnetic field and a magnetic sensor, such as a Hall-effect probe, to sense the magnetic field. The presence of ferromagnetic material, such degraded coating, affects the field distribution and the field measured by the magnetic sensor. Both the eddy current and permeability probe measurements are sensitive to the magnetic condition of the coating and to probe liftoff and tilting effects. The eddy current measurements are also sensitive to electrical conductivity and to the thickness of the coating even when the coating is not degraded. Even though the permeability probe is sensitive to coating thickness after the coating has degraded, because a permanent magnet is used, it is not possible to use different frequencies to control penetration into the coating (skin depth) so as to be able to vary penetration depth into the coating, nor is it possible to profile coating degradation with depth. These variations in measurement capability and sensitivity make it difficult to accurately detect and characterize coating degradation.
Therefore, a nondestructive evaluation technique is needed which can be used in the field to provide an accurate measurement of coating degradation so that the coating may be replaced before damage to the underlying component occurs. There is also a need for a technique that is not sensitive to probe liftoff and tilting effects, electrical conductivity and thickness of the coating. In addition, a technique is needed that allows a profile of coating degradation as a function of depth.